56 lines
1.3 KiB
Kotlin
56 lines
1.3 KiB
Kotlin
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package net.shadowfacts.cacao.util
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import java.util.*
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import kotlin.NoSuchElementException
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/**
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* A linear time algorithm for finding the lowest common ancestor of two nodes in a graph.
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* Based on https://stackoverflow.com/a/6342546/4731558
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*
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* Works be finding the path from each node back to the root node.
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* The LCA will then be the node after which the paths diverge.
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*
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* @author shadowfacts
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*/
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object LowestCommonAncestor {
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fun <Node> find(node1: Node, node2: Node, parent: Node.() -> Node?): Node? {
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@Suppress("NAME_SHADOWING") var node1: Node? = node1
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@Suppress("NAME_SHADOWING") var node2: Node? = node2
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val parent1 = LinkedList<Node>()
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while (node1 != null) {
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parent1.push(node1)
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node1 = node1.parent()
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}
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val parent2 = LinkedList<Node>()
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while (node2 != null) {
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parent2.push(node2)
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node2 = node2.parent()
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}
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// paths don't converge on the same root element
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if (parent1.first != parent2.first) {
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return null
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}
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var oldNode: Node? = null
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while (node1 == node2 && parent1.isNotEmpty() && parent2.isNotEmpty()) {
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oldNode = node1
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node1 = parent1.popOrNull()
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node2 = parent2.popOrNull()
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}
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return if (node1 == node2) node1!!
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else oldNode!!
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}
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}
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private fun <T> LinkedList<T>.popOrNull(): T? {
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return try {
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pop()
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} catch (e: NoSuchElementException) {
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null
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}
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}
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